replaced speedometer driven gear with recommended 40 tooth and still not accurate...

CudaFactHackJob said

To find actual tire diameter, go from the ground to the middle of your back wheel and multiply by 2.

Click to expand...

That's a waste of time. The tire manufacturer's numbers are close enough. A 225/60R15 is 25.6" diameter. Use that number. You won't be able to measure closer than +/- 1/4" anyways. Which is less than 1%.